With an increase in the packing density of LSIs, the required linewidths of circuits included in semiconductor devices become finer year by year. To form a desired circuit pattern on a semiconductor device, a method is employed in which a high-precision original pattern (i.e., a mask, or also particularly called reticle, which is used in a stepper or a scanner) formed on quartz is transferred to a wafer in a reduced manner by using a reduced-projection exposure apparatus. The high-precision original pattern is written by using an electron-beam writing apparatus, in which a so-called electron-beam lithography technique is employed.
An electron beam writing apparatus performs writing with a deflector deflecting an electron beam. Examples of the role of beam deflection performed by a deflector include control of the shape and size of a beam shot, control of a shot position, and beam on/off control (blanking). An output voltage from a DAC (digital-analog converter) amplifier is used to drive the deflector.
In the electron beam writing apparatus, an irradiation time is calculated by dividing, by the current density, the dose (the amount of irradiation) which is set for a writing target pattern. A deflecting voltage is applied from a DAC amplifier to a blanking deflector so that the beam is ON during the irradiation time. The rising and falling edges of the output voltage from the DAC amplifier are not step signals. The blanking deflector needs a finite time to switch the beam ON/OFF. Therefore, the irradiation time is different depending on the number of passes (multiplicity) in multiple writing, resulting in a change in the size of a writing pattern depending on the multiplicity.
By repeatedly writing patterns in an overlying manner, multiple writing aims to utilize the effect of averaging so that an error in pattern position accuracy and an error in connection accuracy of patterns, which is produced in a boundary in a deflection region, are alleviated. In the case where a low sensitivity resist is used and where the amount of irradiation for one pass is large, resist scattering may occur and deflector contamination may occur. To avoid such deflector contamination, multiple writing may be also performed.
To reduce a change in the size of a writing pattern depending on the multiplicity, the electron beam writing apparatus is provided with a shot time offset function of adding an offset time to the calculated irradiation time. In the related art, while the offset time is changed, evaluation patterns are written with multiple different multiplicities, and an optimal offset time is determined from the result of size measurement of the evaluation patterns. However, when the amount of lens excitation is changed in accordance with beam adjustment, an optimal offset time is also changed, hindering improvement of writing accuracy. A method of writing evaluation patterns and calculating an optimal offset time for every beam adjustment may be employed. However, this method has a problem of an increase in downtime of the apparatus.